JP3673200B discloses an apparatus which performs an engine warming-up operation. In this engine warming-up operation, an increase of an engine temperature is promoted and accelerated by increasing an amount of heat loss by retarding ignition timing.
Here, an engine output, which is a work amount of the engine demonstrated per unit time, includes a rotational output, i.e., kinetic energy, on a crankshaft and a heat loss, i.e., thermal energy. Fuel consumption, i.e., fuel consumption rate, may be improved by reducing the heat loss and by increasing a ratio, i.e., shaft efficiency, of the rotational output to an amount of fuel consumption. By performing the ignition timing retarding, it is possible to increase the heat loss, and to promote a warming-up. However, the shaft efficiency becomes worse and the fuel consumption becomes worse.
On the other hand, vehicles, e.g., hybrid vehicles, which has a charge system for generating electric power by using a part of rotational output and charging the generated electric power to a battery is known. JP4300600B discloses a warming-up operation for such a hybrid vehicle. In this operation, the warming-up is accelerated by increasing the engine output. Simultaneously, an amount of increased rotational output caused by increasing the engine output is assigned to generate electric power, and generated electric power is charged to a battery. Therefore, it is possible to promote temperature increase without worsening shaft efficiency.
Points A, B1, C1, D1, and E1 shown in FIG. 3, may be referred to as optimal shaft efficiency points which are combinations of revolution speed of the engine and torque which maximize the shaft efficiency. A line Em shown in FIG. 3 may be referred to as an optimal shaft efficiency line which can be obtained by drawing a line passing through the optimal shaft efficiency points for each engine output level. In the operation, when an amount of temperature increase of the engine required for a warming-up is insufficient, an engine output is increased along the optimal shaft efficiency line Em so that the revolution speed and the torque are adjusted on the optimal shaft efficiency point. Thereby, it is possible to promote temperature increase without worsening the shaft efficiency.